Press molding of sheet metal is performed by driving a punch against the sheet metal while gripping a grip section that is located on the outer periphery of the sheet metal by upper and lower dies of a vertical forming die. After molding a product portion which is located inner side of the ring shaped grip section, a pressed product is obtained by cutting the grip section from the periphery of the product portion.
During press molding, and in particular during draw molding or draw forming, peripheral sheet metal displaces towards the product portion as a result of the pressing process. This phenomenon is termed blank inflow. It is necessary to retain the grip section in order to control the amount of inflow. An appropriate gripping force is applied by the forming die onto the grip section in order to counteract a detaching force towards the center which acts on the sheet metal gripped by the upper and lower dies.
The gripping force on the grip section applied by the forming die is created by forming a tongue and groove section termed a bead on the grip section. The bead is formed using a bead molding section provided on upper and lower dies of the vertical forming die. Pressing operations are simultaneously or thereafter performed on the product portion using the punch with the vertical forming die gripping the bead.
JP10-005889A published by the Japan Patent Office in 1998 proposes a method of forming the bead on the grip section of the sheet metal. According to this method, a bead being trapezoid in cross section and having rounded corners is formed on the grip section. In this prior art method, the tongue section and the groove section are formed as a bead on the grip section by the upper and lower dies of the forming die. Further, when the vertical forming die molds and retains the bead, it is arranged such that only the tongue section of the forming die abuts with the metal plate. Sections other than the tongue section including the groove section of the forming die do not come into contact with the sheet metal.
As a result, even when the clearance between the upper and lower dies of the vertical forming die is not uniform as a result of, deviation of the core, for example, a stable resistance force which acts on the blank when it surmounts the bead or stable blank inflow amount is produced without the necessity to regulate the clearance.